The invention relates to image recording and, more particularly, to a novel image recording process and apparatus wherein latent images are formed in certain metal-salt-containing recording elements by flowing a small imagewise pattern of electrical current therein and then heat processing the recording element to produce a visual image.
In recent years, a large research effort has been directed toward the development of new, as well as the improvement of old, image recording processes. The better known and commercially more successful of these recording processes can be broadly classified as being photographic, thermographic or electrographic or, as being a combination of two or more of these techniques, for example photothermographic. The terms photographic, thermographic and electrographic as used herein refer to recording processes in which the phenomena of light, heat and electricity, respectively, are used for the purpose of recording and reproducing patterns in viewable form. Each of the known image recording processes has its own advantages in particular applications but suffers from disadvantages which limit its usefulness in other applications. For example, conventional photography has the disadvantage of requiring chemical development procedures, thermography requires an imagewise heating procedure, and xerography, one form of electrography, requires a mechanical dust pattern transfer procedure.
It is known that images can be formed in certain recording materials by passing an electrical current therethrough and considerable effort has been expended in investigating this electrogaphic image recording technique. For instance, K. S. Lion et al in a report entitled "Investigation in the Field of Image Intensification, Final Report," in Air Force Cambridge Research Laboratories AFCRL 64-133, Jan. 31, 1964, Contract No. AF 19(605)-5704 disclose an electrographic process in which the recording element comprises a conventional light-sensitive photographic emulsion which is positioned adjacent to a photoconductive layer. Upon applying a uniform electric field across the photoconductive and photographic layers and simultaneously imagewise exposing the photoconductive layer to a light pattern, an imagewise current is produced in the photographic layer. This imagewise current flow, in turn, produces a chemically developable latent image in the photographic layer, which is more intense for a given light exposure than an image produced by imagewise exposing the photographic layer directly. The amplification is particularly pronounced when the incident light is of a color to which the photoconductor is responsive, but to which the photographic emulsion is not responsive.
While the Lion et al. recording technique presumably offers advantages in the form of increased sensitivity, it suffers the disadvantages associated with the use of a light sensitive, chemically developable recording layer. Moreover, the production of a latent image in a conventional light sensitive photographic emulsion requires a substantial current flow in the emulsion; hence, a relatively lengthy exposure time with low current flow or a high current flow with a short exposure time is required.
Another approach to the production of visible images is disclosed in U.S. Pat. No. 3,138,547 issued in the name of B. L. Clark. This approach involves the use of a light-insensitive, electrosensitive recording layer composed of particles of a reducible metal compound capable of electrical reduction in situ. The recording layer is disposed on an electrically conductive backing, and recording is effected by contacting the layer with an electrically charged stylus, thereby causing a current to flow through the layer. This current is sufficient to reduce the particulate metal compound, in the dry state, to provide a visible image.
A drawback of the recording process disclosed by Clark is that it incorporates no gain or amplification. For each reduction event leading to an increase in density of the final image, an additional quantity of electronic charge flowing through the recording element must be provided. Thus, relatively high current densities must be provided in order to produce a visible image in a reasonable period of time.
Still another recording technique is disclosed in U.S. Pat. Nos. 2,798,959 and 2,798,960 issued July 9, 1957 in the name of A. J. Moncrieff-Yeates. In accordance with the teachings of these references, a photoconductive material and a heat sensitive material are interposed between and in electrical contact with a pair of electrodes. An optical image is projected on the photoconductive material while a voltage is applied across the electrodes. The flow of electric current heats the photoconductive material, the heating effect in each increment of area being a function of the amount of current flowing, the resistivity of the photoconductive material and the intensity of the illumination. The heat image thus produced in the photoconductive material changes the heat sensitive material to form a permanent image therein.
One disadvantage of the Moncrieff-Yeates recording process is that high current flows are required in the photoconductive material in order to produce sufficient quantities of thermal energy for image formation. Furthermore, this recording process, in common with the Clark process requires an incremental increase in current flow for each incremental increase in density of the final image.
An image recording process which incorporates gain is disclosed by Tokumoto et al. in U.S. Pat. No. 3,425,916. According to this process, chemically-developable nuclei are formed in a reagent layer by imagewise exposing the layer to a relatively minute current flow. Unlike direct print-out image recording processes, such as mentioned above, the current flow itself need not be sufficient to produce a visible reaction in the reagent layer in situ. Rather, it need only be sufficient to produce nuclei which, during a subsequent chemical development step, can be amplified to produce a visible image.
While the aforementioned Tokumoto et al. process requires relatively low current flow to produce a developable latent image, the overall process requires that the recording material be moistened during the latent image or nuclei forming step. Moreover, the recording element on which the nuclei forming process is carried out requires chemical liquid development in order to intensify and thereby render visible the current produced nuclei. Furthermore, once developed, the visible image must be stabilized by washing and fixing, as in ordinary photographic processes. For these and other reasons, this process has not, to date, enjoyed substantial commercial use.
Another electrographic image recording process which incorporates gain is disclosed by Tokumoto et al. in United Kingdom Pat. No. 1,275,929. In this process, a latent image is formed by applying an imagewise electric current to a dry and conductive recording sheet formed of a conductive powder and an image forming component in a binder. The recording sheet is then heated in the presence of a redox system which includes a compound including at least one metal selected from nickel, cobalt, zinc, chromium, tin and copper to produce a visible image with the image forming component.
One disadvantage of this process is that it requires relatively large current flows (i.e., .ltoreq.1 milliampere/cm.sup.2) through the conductive recording sheet for short times. Consequently the production of relatively high charge density levels (i.e., .ltoreq.1 millicoulomb/cm.sup.2), are required for latent image formation. In certain electrographic image recording applications, the use of a conductive recording material and/or the production therein of a charge density of 1 millicoulomb/cm.sup.2 or greater is either impossible or undesirable within a commercially practical exposure period. One example is the use of electrosensitive recording materials with sources of activating electrical energy, such as corona discharge devices or electrostatically charged devices, that do not develop a high electron current and cannot, therefore, produce a high charge density level in a short exposure period. Another example is the use of electrosensitive recording materials to detect electromagnetic radiation by sandwiching such a recording material with an electrophotographic photoconductor. To produce an imagewise current flow through the recording material, the resistivities of the photoconductor and the recording element must be matched within a predetermined range. Presently existing electrophotographic photoconductors are high impedance, low current devices. Therefore, if the recording material is highly conductive relative to the photoconductor a high current flow will be produced in the sandwich resulting in photoconductor breakdown, which breakdown in turn prohibits the formation of a latent image. One proposed use of an electrophotographicphotoconductor-electrosensitive recording material sandwich is medical radiography. In this application it is desirable to subject the patient to as small a dosage of X-ray radiation as possible. The recording material, therefore, must be capable of forming a latent image with the low charge density produced therein by the brief X-ray exposure of the high impedance, low current output photoconductor.